These stairlifts are often used to convey a person who has difficulties with walking along the staircase. The rail extends in said situation along the staircase. In practise, the rail often does not extend along a straight line, but has one or more bends. These bends can be in any direction, which means the bends can be in the vertical direction and/or in the horizontal direction. The rail can therefore have a varying angle relative to a horizontal plane and/or a varying angle relative to a vertical plane.
To keep the load carrier, such as a chair or wheelchair platform, in a desired orientation relative to a horizontal plane, the stairlift is provided with a rotation device configured to rotate the load carrier about a horizontal axis. In most cases, the load carrier is kept in a horizontal orientation while the carriage is moved along the parts of the rail with a varying angle.
In the known stairlifts dangerous situations may arise when the rotation device (for whatever reason) stops functioning. This can lead to the situation that the load carrier falls down with a pivot movement about the horizontal axis.
WO2012/12086 discloses in FIG. 2 a levelling system having a levelling motor which drives a first worm gear. The first worm gear engages a worm wheel in order to rotate it. A locking mechanism is provided as a safety back-up arrangement. The locking mechanism comprises a second worm gear which is rotated synchronically with the worm wheel. The second worm gear does not engage the worm wheel. There is a clearance between the second worm gear and the worm wheel, due to which the second worm cannot drive the worm wheel. If there is a failure at the levelling motor or the first worm gear, the second worm gear acts as a locking mechanism to lock the worm wheel. This means that the first worm gear only has the function of driving the worm wheel and the second worm gear only has the function of locking the worm wheel.